Conventionally, there is an image forming apparatus which forms an image using image forming material that can be erased (decolored) by prescribed erasing processing. In addition, in order to protect an image (security image) required for protection of duplication prohibition and so on, an image forming apparatus may form an image of a specific pattern such as an electronic watermark along with the security image. However, actually, erasable image forming material has not been erased completely, and thereby an erasing residue may be generated. When the specific pattern is erased by the erasing processing, and a part of the security image remains in the replicable state, the part of the security image might become in the illegally replicable state.